This invention relates to phased array antennas, and, more particularly, to ultra-wideband phased array antennas for radio frequency and optical beam forming.
Ultra wideband systems are known in the art as those systems that transmit and receive electro-magnetic energy over a wide frequency band where the instantaneous fractional bandwidth of the system exceeds 25%. Some of the advantages of ultra wideband (UWB) systems are: lowered probability of intercept of transmissions; reduced multipath fading and radio frequency interference problems; and enhanced target recognition performance. For transmission of UWB signals, a beamformer must be able to form and direct a beam that retains the full bandwidth of the UWB signal.
Conventional beamformers operate with instantaneous bandwidths generally less than 25%. They employ a combination of phase shifters, delay lines and antenna subaperture configurations to minimize the pulse shape distortion and beam broadening that are usually collectively referred to as xe2x80x9cbeam squint.xe2x80x9d For beam squint to be zero, the pulse envelopes emitted from each radiating element must coincide at the receiver, and the carriers (if present) must all be in phase.
The prior art for wideband and ultra wideband systems employs combinations of phase shifters, fixed non-tunable delay lines, and antenna subaperture geometries. For instantaneous fractional bandwidths close to 100%, the subaperture size needed to achieve maximum acceptable levels of beam squint approaches the radiating element spacing. That is, only one element per subaperture can be accommodated, so that the use of subapertures is no longer a meaningful tool at UWB. Systems with phase shifters setting the phase at a single frequency are also not broad-band and cannot be used in the UWB portion of a system such as at the radiating element.
The prior art discloses the use of fixed bulky delay lines as a feasible component for ultra wideband beamforming system. Such a system is disclosed by Newberg et al., in U.S. Pat. No. 5,475,392, xe2x80x9cFrequency Translation of True Time Delay Signals,xe2x80x9d issued Dec. 12, 1995. Newberg et al. disclose providing each antenna element in a phased array with a separate frequency translated transmit signal. The frequency translated transmit signal is created by mixing a true time delayed beamsteering signal with a phase shifted or true time delayed local oscillator signal. Delay lines or other true time delay circuits are used to provide the delay required for the true time delayed beam steering signal.
However, delay lines have severe implementation problems when applied to UWB systems. Since subapertures cannot be used, a beamformer must have a dedicated delay line feeding each radiating element. This results in a large number of delay lines. Since phase shifters, inherently narrow beam devices, cannot be used, each beam scan angle must be established by a separate time-delay state. Since the scan angle separations must be on the order of a beam width for full coverage in two dimensions, many time delay states are needed for control. Thus, for an antenna with a large number of elements and/or a narrow beam, the number of delay lines and delay states required become prohibitively large. Even a moderate resolution system implemented in this way would be extremely complex and expensive.
Radio Frequency (RF) mixing feed systems such as the Heterodyning Rotman antenna as disclosed by Lee et al. in U.S. Pat. No. 5,861,845, xe2x80x9cWide-Band Phased Array Antennas and Methods,xe2x80x9d issued Jan. 19, 1999 can be effective at producing beams with moderate amounts of beam squint at bandwidths in the low range of UWB (i.e., bandwidthsxe2x89xa625%). However, at the higher bandwidths, the heterodyning scan range must be held extremely narrow in order to keep beam squint within tolerable levels. This in turn requires that a large number of true-time-delay ports (sets of physical delay lines) be used. Thus the complexity of having a large number of delay line sets and the problems of switching between them as a target is tracked become very cumbersome with such higher bandwidth UWB beamformers.
In light of the above discussion, there exists a need in the art for a squint-free, continuously scanned ultra wideband phased array antenna beamformer. A system providing such ultra wideband beams should be low cost and capable of being fabricated from off-the-shelf components.
It is an object of the present invention to provide a method and apparatus for forming ultra wideband phased array antenna beams with no beam squint. It is a further object of the present invention to provide a low cost ultra wideband beamforming system that can preferably be fabricated from off-the-shelf components.
The present invention uses phase shift key modulation to impress data information in the form of a phase shift xcex94"PHgr"D onto the phase of a sine wave of frequency fp. The phase xcex94"PHgr"D=2xcfx80fpxcex94T corresponds to the time delay xcex94T required in a pulse position modulation format. The data-bearing sine wave is then split into N transmission lines where each undergoes an additional antenna scanning phase shift xcex4xcfx86Sn=2xcfx80fpnxcex4t, where xcex4t is the required inter-element time delay for scanning the antenna to angle xcex80, and integer n is an index specifying the transmission line. Each line signal then passes through a non-linear element which converts the sine waves into short pulses, with each pulse appearing at a peak of the sine wave. The pulses in each line are then sent to a corresponding antenna element where they are emitted.
Pulses emitted by antenna element xe2x80x9cnxe2x80x9d have total delay times TTOT=xcex94T+nxcex4t corresponding to the sum of the data and the scan angle delays. In the antenna far-field at scan angle xcex80, the progressively increasing scan delays nxcex4t between elements are canceled out by the different propagation distances to the far-field. As a result, the electric fields from all elements during the pulse interval coincide and vector sum at the receive antenna with a common delay time xcex94T corresponding to pulse position modulated data. The vector summed signal is then demodulated by the receiver and the pulse position modulation data is recovered.
The phase shifters and pulse formers of the present invention can be provided by commercial-off-the-shelf-components or other devices well known in the art. Such devices provide for creation of pulses on the order of ten to one hundred picoseconds. The antenna elements are deployed in antenna arrays also well known in the art.
One embodiment of the present invention provides an ultra-wideband beamformer for transmission of data symbols by a pulsed beam comprising: a sine wave generator; a modulator modulating the sine wave with the data symbols; a plurality of phased output paths, each providing a pulsed beam wherein each phased output path comprises: a phase shifter; a pulse former; and a radiating element.
Other embodiments of the present invention provide for modulation of data symbols by suppressing one or more phases of the generated sine wave. Sine wave suppression can also be used to encode the pulsed output provided by the system. Sine wave suppression can be performed by a separate modulation element or can be performed by the pulse former used within the invention.
Another embodiment of the present invention provides a method for forming an ultra-wideband phased array beam comprising the steps of: providing a stream of data symbols; generating a sine wave; modulating the sine wave with the stream of data symbols; providing the modulated sine wave to a plurality of phased output paths; phase shifting the modulated sine wave in each of the phased output paths to provide the delay required for a beam scan angle, said phase shifting providing a delay required to provide a beam scan angle; forming a pulse from each phase of each delayed modulated sine wave; and sending the pulse to a radiating element.
Optical beamforming is also provided by the present invention. The components used for electrical beamforming are combined with electro-optical components to provide an ultra-wideband optical beamforming system. Optical beamforming systems formed from gain switched laser diodes provide ultra-wideband systems with optical pulses as short as three picoseconds. Optical beamforming systems formed from electro-absorption modulators provide pulses as short as 10 picoseconds at up to 20 gigapulses per second.